This invention relates to a new method to detoxify municipal sewage sludge contaminated with heavy metals. More particularly, this invention relates to a new method to detoxify municipal sewage sludge so that the heavy metals contained therein could meet or exceed the sludge quality standards established by the United States Environmental Protection Agency for safe land application.
With reference to the Federal Register dated Feb. 6, 1989, and cited as Volume 54, Number 23 commencing on Page 5754, the U.S. Environmental Protection Agency will implement stringent regulations to protect public health and the environment from any anticipated adverse effects of pollutants contained in sewage sludge. In brief, these unprecedented regulations which are targeted for implementation in October, 1991, establish requirements for the final use and disposal of sewage sludge. The Association of Metropolitan Sewage Agencies (AMSA) considers these regulations a radical departure from current sludge use and sludge disposal practices at existing publicly owned treatment works (POTW's). AMSA believes that, in many instances, existing sludge management practices at POTW's would not be in compliance with these proposed regulations.
Sewage sludge is the residue from processed municipal sewage and it typically contains more than 90% water, colloidally dispersed solid particles (some of which are fragile) and dissolved substances. Although the chemical and biological constituents of sludge depend upon the composition of the wastewater entering the POTW and the subsequent treatment processes, typically, these constituents are heavy metals, disease-causing pathogenic organisms such as bacteria, viruses and parasites, volatile organic solids, nutrients, humates and/or industrial waste.
Metals contained in sewage sludge pose a severe threat to human health and the environment. Approximately 7.5 million dry tons of sludge are produced in the United States annually. Approximately 20% of this sludge is applied to land for its organic and nutrient value; approximately 40% is disposed in municipal landfills; approximately 5% is disposed by ocean dumping; and approximately 20% is incinerated. All of these methods of use and disposal adversely affect the environment because the cumulative concentration of these metals as they recycle into the environment through these methods can have a toxic effect in the food chain and, subsequently, on human health. Toxic levels of heavy metals can accumulate in human kidneys, liver and other organs of the body causing functional disorders which can be lethal.
In the past, processes have been developed to remove heavy metals from sewage sludge. A typical method of heavy metals removal from sewage sludge utilizes standard acidification techniques. Although acidification is effective in leaching (also commonly referred to as "solubilizing") the more soluble heavy metals from sludge, some of the predominant heavy metals in sewage sludge are not amenable to acid dissolution.
Hayes et al. in their U.S. Pat. No. 4,370,233 disclose a method for chemical detoxification of anaerobically-digested organic sludge containing toxic heavy metals in insoluble form. This method includes raising the oxidation reduction potential (hereinafter sometimes referred to as "ORP") of the sludge to above +300 millivolts (hereinafter referred to as "mv") and maintaining this condition for a period of 6 to 12 hours. During this time period, the sludge is oxidized making the heavy metals more conducive to chemical leaching. Immediately thereafter, the sludge is acidified to a pH range between 1.0 to 3.0 for an additional period of 6 to 12 hours by the addition of a concentrated acid such as sulfuric acid or hydrochloric acid while continuing to agitate the sludge and while maintaining a prescribed temperature range and ORP above +300 mv by aeration. Hayes et al. rely upon the principle that if sludges can be maintained at an elevated temperature and ORP for a prolonged period of time prior to acid treatment, a shift in heavy metal speciation will occur toward metal precipitate forms that are more rapidly solubilized upon acidification.
Several drawbacks seem apparent with the Hayes et al. method of heavy metals detoxification of sewage sludge. First, residence time is long; the minimal combined time period for oxidation and acidification is 12 hours. Such a long residence time suggests a need for extremely large tankage to facilitate chemical processing. Second, oxidation and acidification occur sequentially which also contributes to the lengthy time period for this method to be effective. Third, nothing disclosed in the patent suggests that an oxidation reduction potential much above +300 millivolts can be achieved. Fourth, temperature maintenance is an important feature which adds to the cost of operations. Fifth, it is preferable to elevate the temperature and ORP of the sludge for prolonged periods of time prior to acidification. Again, this two-staged process requires a relatively long time period to be effective and there are added costs associated with elevated temperatures as well as maintenance thereof.
It is from these considerations and others that the present invention has evolved.